Method of forming sheets from perfluorochlorocarbon plastic



Nov. 11, 1952 RuBlN 2,617,149

METHOD OF FORMING SHEETS FROM PERFLUOROCHLOROCARBON PLASTIC Filed Dec. 31, 1949 PLATEN l5 REFLECTOR v GAUGE BLOCK FIXED 2 HEATING P AT N UNIT L E MOLDED POLYMER INVENTOR. LOUIS c. RUBIN BY 05W ATTORNEYS Patented Nov. 1 l l 952 f UNITED STATES PATENT OFFICE METHOD OFFORMING SHEETSFR'OME PER- FLUOROCHLOROCARBON PLASTIC;

Eouis. G1. Rubin, West Caldwell;. N J1, assignor; to The M. W. Kellogg Company, Jersey'City, Ni J a. corporation of' Delaware- Application December 31, 1949, Serial No. 136,199:

8' Claims. (CI; 18-55) This: invention relates to the treatment. of great care and a novel method. or molding. are perfiuoroch-lorocarbon plasticsl. In. one aspect, required informing. molded sheets. of these. plastheinvention relates to the treatment of polytics-inasmuch as they tend tochange their physitrifiuorochloroethylcne plastics. In another ascal characteristics, to varying degrees. under. the pect the invention relates to the molding of 5 influence of the molding, temperature. polymers of trifiuorochloroethylene informing It is an objectof this inventionto. provide an sheets-of such material withouttheuse or dies. improved. methodof. molding. polymers of tri- The preparation of the monomer,. trifluorofluorochloroethylene in. forming sheets without chloroethylene, is accomplished by dechlorinatthe use of dies.

in-g Freon 1-13- (Ll-2 trifiuorotrichloroethane), 30 Various other objects and advantages of the under suitable. conditions oi dehalogenation in presentinvention may beapparent to those skilled the presence of a'solvent, such as methyl alcoin. the art from the. accompanying description hol, and ametallic dehalogenating: agent, such and disclosures a's zinc' dust, to produce an eflluent comprising P The accompanying. drawings of{ the present the monomer trifluorochloroethylene, including invention illustrate the techniques: of operation unreacted trifiuorochloroeth-ylene' and solvent. in. molding polymers of trifluorochloroethylene This eflluent is nextipassed tea-suitable fractional without. the use of. dies. Figure I is a, front view, distillation system which; substantially pure partlyin. cross-section, 0t apparatus employed trifluoroclrloroethyleneis recovered as-arelatively for molding. the polymer and determining, when low boiling fraction. 39 plastification is complete,. as hereinafter dis.- Themonomer thus obtained is polymerized cussed, Figure 2 isagraph used for determining undersuitable polymerization conditions, with or the required gauge block thickness to obtain without the presence of a suitable catalyst Or sheets of the trifiuorochloroethylene molded promoter; Such conditions may comprise the polymer of desired thickness, also hereinafter use of a suitablecatalyst comprising: an organic discussed, peroxide, such as bis-trichloroacetyl peroxide In l'noldi'ngv sheets of plastic material, a. suitable preferably dissolvcdin-asuitable solvent,.such as qu y o t plastic 00111190111161, p p d n trichlorofluoromethane,ata temperature between granular or powdered form, in accordance with about 20 C. and. about 25 0., and preferably the usualpractice, is placed on a polished surface at a temperature of about. -16" 0, m, bemperao which is heated to a suitable molding temperature. ture of about l6 C. the polymerization of tri- Gauge blocks, of a, thickness appropriate to the fluorochloroethylene to the solid polymer in a thickness desired, in the molding sheet ar placed suitable yieldisaccomplishedin about seven days. on the polished. surface at the sides thereof. A At elevated temperatures, less time is required DOIi'Shihg Plate s then p d above t p s to complete. the polymerization. After the de- 57.; mass in position. to bear on the gauge blocks siredextentof polymerization, the resulting polye presseddflwnr Molding is thencarried- 011i? merization reaction mixture is removed from the in a, Suitable spolymerization zone and the polymeric product In. accordance with this invention, sheets oi is recovered from the solvent for the catalyst or the polymersof trifluorochloroethylene are mold.- polymerizing agent. edin. the above manner without the use of dies,

The polymers of trifiuorochloroethylene possess by carefu sel cti n of m n temp rature, i certain desirable, physical and chemical characth range of F- to 5( h m ld mteristics', with four-fifths of the weight being supperature bein Selected by reference to phy c l plied by the halogens, fluorine and chlorine. r p rties and/0r m l cular w ht of th p ly- These polymers are colorless, transparent, and 413 l Preferably th gauge b a ome hat have been found to possess a high chemical thicker than the desired sheet thickness. to allow stability, with no effect being observed on the for shrinkage-upontsubseq e o polymer after prolonged exposure to concentrated Polymers of trifiuorochloroethylene of varying sulfuric, hydrofluoric and hydrochloric acid, mol cular w i ht may b produ d by the m h strong caustic, fuming nitric acid, aqua regia, and 50 described: above, by suitable control of conditions other vigorous oxidizing materials. The polymer polymerization. In molding sheets of. such is hard but not brittle and is fiowable at temperap l m rs. ina c rda e w h t s invention, h tures above about 500 F. In addition, the polymolding temperature is carefully selected by refmer is flexible and resilient, and is not wetted erence, directly or indirectly, to the molecular by water or affected by high humidity. However, weight of the polymer tobe molded. Tempera- 3 tures in the range of 415 F. to 560 F. are satisfactory, but best results are obtained by selecting a molding temperature within this range which is the optimum temperature for the particular polymer being molded.

It is impractical to ascertain the molecular weight for each polymer to be molded under normal circumstances. Accordingly, a simple test has been devised which provides a direct indication of the proper molding temperature. This test comprises measurements of the temperature at which no tensile strength is exhibited under the conditions of the test. This test is applied to standard or test pieces of the plastic material. This temperature is referred to as the no strength temperature or no strength temperature value, and is abbreviated N. S. T. Accordingly, the N. S. T. value serves as a useful guide in the molding of plastics, and has particular applicability to plastics composed essentially of polytrifiuorochloroethylene.

The measurement of the N. S. T. value is carried out in suitable apparatus on a strip of the pastic measuring 2" by by Conveniently such strips may be notched or grooved in a straight line across one large surface at the center transversely to the longitudinal axis. The notch or groove is made perfectly straight and extended to a depth of ,4, 'The "strength test is then applied to the point of minimum crosssection. The sample is suspended vertically, with a small weight (approximately 4 long), suspended from the lower end of the test strip on a fine wire. The above-mentioned weight is so adjusted that the total weight from the notch down is 0.5 gram. The test piece is suspended in a suitable chamber whose internal temperature is brought up to 410 F. The temperature is then raised at a rate of 2.7" F. per minute until the sample pulls apart at the notch. The N. S. T. value is the temperature at which the test strip is pulled in two. This determination is not highly sensitive to small variations in test strip thickness (:0003"), however care must be taken to cut a sharp clean notch of uniform depth. Differences of 10 'F. are normally considered significant.

In carrying out the molding of polytrifluorochloroethylene under the aforementioned conditions of temperature and pressure, the optimum operating conditions are determined by reference to the particular N. S. T. value of the polytrifluorochlcroethylene plastic treated. The N. S. T. value of the polytrifluorochloroethylene plastics vary from about 220 C. to about 350 C. and these polymers may be satisfactorily treated under the above range of operating conditions. In general, it has been found that as the N. S. T. value increases the molding temperature should also be increased (if the pressure is the same) to insure the best results. An operating temperature range between about 470 F. and about 520 F. is preferred with a compacting pressure preferably bet een ab ut 5 0 and about 1,5 0 pounds er square inch, although pressures as high as 20,000 pounds per square inch may also be employed. Thus a polytrifiuorochloroethylene plastic having an N. S. T. value of 240 C. can be satisfactorily pressed within the peferred compacting pressure range between about 500 and about 1,500 pounds per square inch, at a temperature between about 470 F. and about 485 F.', while a polymer having an N. S. T. valve of 270 C. is satisfactorily pressed at a temperature between about 485 F. and about 500 F., within the preferred pressure range. Similarly, a polymer having an N. S. T. value of 300 C. is satisfactorily pressed at a temperature between about 500 F. and about 510 F. within the preferred pressure range. The polytrifiuorochloroethylene plastic having a rela tively low N. S. T. value such as 240 C. flows easier at the molding temperatureand produces a relatively harder finished sheet, while the polymers having a relatively high N. S. T. value such as 300 C. are more viscous at the molding temperature but produce a more flexible finished sheet.

The particular pressing time required for completely converting the stock and forming the finished sheet under selected operating conditions of temperature and pressure and determined by the respective N. S. T. values, is based upon the time required for plastification to be complete when the press is closed and the molding cycle is started. To determine when plastification is complete a simple test has been devised, as shown in Figure 1 of the accompanying drawing which is a front view partly in crosssection, in which an electric light I0 equipped with a reflector H is placed in the rear of the press (not shown) and level with the surface of the fixed polishing plate i2 equipped with heating unit l3. By looking through the stock or molded polymer I l with movable platen I5 (equipped with heating unit 16) bearing thereon from the side opposite to the position of the light It), a color change in the stock M will be noticed as its temperature rises. At first the stock assumes a light pink color which gradually changes to the color of white light when the point of complete conversion is reached. As previously indicated the plastic sheet I4 produced by the aforementioned molding operation after the cycle is complete, is slightly greater than the desired thickness. Such procedure is made necessary by reason that subsequent cooling or quenching of the hot molded sheet tends to bring about a reduction in thickness. Accordingly, the use of gauge blocks 11 slightly thicker than the desired thickness of the final finished plastic sheet, is indicated. In this respect, it has been found that a definite fixed relationship exists between gauge block thickness and the desired thickness of the final finished plastic sheet. Reference is therefore had to Figure 2 in the accompanying drawing which shows a graph obtained by plotting the desired sheet thickness, as abscissae, against the required gauge block thickness, as ordinates. It will be noted from the graph that the relationship existing between gauge thickness and desired sheet thickness is a straight line function in which the thickness of the gauge block selected for the molding operating is about 1.25 times greater than that of the desired thickness of the final plastic sheet. The followin table indicates typical gauge block thicknesses required to produce a finished plastic sheet (after the cooling or quenching operation) which has the desired thickness.

i I Gauge Block- Finished Shcct- Inches Inchesl o. 05:) t 0. 040 1 o. i 0. use i o. 1.50 1 o. 120

0. 20:1 0. i 1 o. 250 I 0. 200 1 1 o. 300 i 0. 240 g It is, therefore, possible to calculate intermediate, higher or lower thicknesses of the re quired gauge blocks either from the graph or by applying the formula in which the required gauge block thickness shouldbe about 1.55 times greater than the desired thickness of the final plastic sheet after the cooling operation.

Inthe description of the operating conditions for the above-mentioned molding of'sheets without a die, the stock to be treated has been referred to as a plastic com-posed essentially of polytrifluorochloroethylene. It should be understood, however, that fillers and plastic compositions other than the polymers of the present invention which do not materially affect the characteristics of pol-ytrifiuorochl'oroethylene may also be incorporated in the plastic material treated, and that the treatment of such compositions is also within the scope of this invention. In this respect, it has also been found desirable to incorporate plasticizing agents with the polytrifluoroch'loroethyl'ene plastic to be molded, employing such plasticizers as polytrifluorochloro ethylene itself but in an oily or waxy state. When incorporating such plasticizing agents with the polytrifiuorochloroethylene plastic to be molded, it is possible to employ the lower temperatures within the aforementioned ranges for a given N. S. T. value of polytrifluorochloroethyl ene. From an economic standpoint, it is generally preferred to conduct the molding operation at as low a, temperature as possible.

In carrying out the aforementioned. molding of sheets of polytrifluorochloroethylene plastics,

conventional apparatus for molding plastic sheets without a die is empoyed which is familiar to those skilled in the art, and since the novelty of the present invention does not reside in the molding apparatus employed, further, description thereof is unnecessary .,-It--ispreferred, however, that the platens'be'electrically heated. The polishing plates of the platens are preferably highly polished stainless steel plates or thin gauged ferro-type plates having a thickness of approximately 0.020", reinforced with heavy gauge steel backing plates, having a thickness of approximately 0.050".

In conducting the moldingoperation, a previously determined weight of the polytrifluorochloroethylene plastic stock, preferably in granular form, is placed on the polished, surface of the platen. It is preferred that this polished plate be first brought up to a temperature between about 225 F. and about 300 F., before the actual molding cycle is begun, in order to avoid possible abrasion of the surface of the polishing plate by contact with the relatively rough polytrifiuorochloroethylene plastic granules, and also to reduce the time required to plasticize the polymer in the actual molding operation. If so desired, the stock may be separately preheated in an oven at a temperature substantially below the conversion temperature and it is desirable to hold the polished surface at. such temperature. for about an hour before the actual molding cycle. is begun.

With appropriate gauge blocks placed on two sides of the polishing plate upon which the granular mass of the plastic is retained, a second polishing plate is brought to bear upon the granular mass, with the upper plate so aligned that it will bear upon the gauge blocks when the press is closed. The press is slowly closed at the rate of about 0.2" per minute at the selected 6 molding temperature, with proper pressuremaintainecl within the aforementioned range, which is preferably between about 500 and about 1,500 pounds per square inch. After conversion is complete the plastic-sheet may be removed from thepress.

As previously indicated the sheet thus obtained is slightly greater than the desired thickness and must be positively cooled in order to reduce this thickness. While cooling of this sheet may be that which is caused bycontact of the sheet with the atmosphere, it is desirable however to positively cool this sheet by quenching in water. Such cooling may be eiiected either by actually immersing the sheet in water or transferringthe assembly 0- the-aforementioned upper and lower polishing surfaces to cool press platens. In the latter case, sufiicient pressure is employed in bringing the cooled pol-ishing plates in contact with the gauge blocks to maintain a suitable pressure within the aforementioned operating range.

The aforementioned rapid cooling of the formed plastic sheet is desirable in order to limit crystallization. Such limitation of the degree of crystallization will affect the physical prperties of the molded sheet produced from th polytrifluorochloroethylene plastic. Thesaprbperties have been found to vary from those of an amorphous material to those of a crystalline material.

The amorphous material is tran parent, softer,

tougher and more flexible than the crystallized material. 7

pearance and hard and resistant .to distortion. The degree of crystallization is determined primarily by the rapidity with which the molded polymer sheet is cooled to approximately 300 F. or below, from its transition temperature. The,

. melt viscisity of. the polymer to be molded effects the rate of crystallization, so that the higher the N. S. T. value, the lower the rate of crystallizaion. Similarly, when the N. S. T. .value is lowcred, the rate of crystallization will be more rapid so that the cooling. operation should bemore rapid if it. is desired to avoid or limit crystallization. For example, quenched polytrifiuoroch-loroethylene plastic havingan N. S. T.v value. of 22050., while flexible and useful. as. a film. at room, tem- Iperature will crystallize fairly rapidly'a't. temperatures as low as 250 F. and become brittle. Quenched polytrifluorochloroethylene. plastic with. an N. S. T. value :of 270 C. will. show com- ,paratively little change on heating. to. the. same temperaturev for prolonged periods; and even .if crystallized by heatingat. relatively higher temperatures, still retains its toughness-..

While a particular embodiment. of the. present invention has been described for purposes of illustration, it should be understood that various modifications and adaptations thereof, which will be obvious to one skilled in the art may .be made within the spirit of the invention without. departing from its scope.

I claim:

1. A method for forming sheets without. a. .die from a thermoplastic polymer of trifluorochloroethylene which comprises placing, the plastic between pressing platens containing a gauge block, compressing said plastic between said pressing platens under suflicient elevated pressure to close said platens against said gauge block at an elevated temperature suhicient to mold said plastic into a sheet, after closing said platens against said gauge block to form the molded sheet maintaining the elevated temperature and The latter tends 'td be .milky"in\ap- F. and about 520 .said plastic between said sheet, after closing pressure at least untila color change indicative of plastification is noted, and thereafter cooling said molded sheet.

2. A methodfor forming sheets without a die from a thermoplastic polymer of trifiuorochloroethylene having an N. S. '1. value between about 220 C. and about 350 C. which comprises placing the plastic between pressing platens containing a gauge block, compressing said plastic between said pressing'platens at a pressure between about 500 and about 20,000 pounds per square inch to close said platens against said gauge block at a temperature between about 415 F. and about 560 F. to mold said plastic into a sheet, after closing said platens against said gauge block to form the molded sheet maintaining said temperature and pressure at least until a color change indicative of plastification is noted, and thereafter cooling said molded sheet.

3. A method for forming sheets Without a die from a plastic composed essentially of polytrifiuorochloroethylene having an N. S. T. value between about 220 C. and about 350 C. which comprises placing the plastic between pressing platens containing a gauge clock, compressing ,said plastic between saidpressing platensat a pressure between 500 and about 1500 pounds per square inch to close said platens against said ge block at a temperature bet een about 415 F. and about 560 F. to mold said plastic into a sheet, after-closing said platens against said gauge block to form the molded sheet maintaining said temperature and pressure at least un- 'til a color changeindicative of plastification is noted, and thereafter cooling said molded sheet. 4. A method for forming sheets without a die from a plastic composed essentially of polytrifluorochloroethylene having an N. S. Ti'value betweenlabout 240 C. and-about 350 C. which comprises placing the plasticbetween pressing platens containing agauge block, compressing said plastic between said pressing platens at a pressure between 500 and about 1500 pounds per square inch to close said platens against said gauge block at a temperature between about 470 F. to mold said plastic into a sheet, after closing said platens against said gauge block to form the molded sheet maintaining said temperature and pressure at least until a color change indicative of plastification is noted, and thereafter cooling said molded sheet.

5. A method for forming sheets without a die from a plastic composed essentially of polytri- .fiuorochloroethylene having an N. S. '1. value between about 2=l0 C. and about 300 C. which comprises placing the plastic between pressing platens containing a gauge block, compressing pressing platens at a pressure between 500 and about 1500 pounds per square inch to close said platens against said gauge block at a temperature between about 415 F. and about 560 F. to mold said plastic into a said platens against said gauge block to form the molded sheet maintaining said temperature and pressure at least until a color change indicative of plastification is noted, and thereafter cooling said molded sheet.

6. A method for forming sheets without a die from a plastic composed essentially of polytrifiuorochloroethylene having an N. S. T. value of about 240 C. which comprises placing the plastic between pressing platens containing a gauge block, compressing said plastic between said pressing platens at a pressure between 500 and about 1500 pounds per square inch to close said platens against said gauge block at a temperature between about 470 F. and about 435 F. to mold said plastic into a sheet, after closing said platens against said gauge block to form the molded sheet maintaining said temperature and pressure at least until a color change indicative of plastification is noted, and thereafter cooling said molded sheet.

7. A method for forming sheets without a die from a plastic composed essentially of polytrifiuorochloroethylene having an N. S. T. value of about 270 C. which comprises placing the plastic between pressing platens containing a gauge block, compressing said plastic between said pressing platens at a pressure between 500 and about 1500 pounds per square inch to close said platens against said gauge block at a-temperature between about 485 F. and about 500 F. to mold said plastic into a sheet, after closing said platens against said gauge block to form the molded sheet maintaining said temperature and pressure at least until a color change indicative of plastification is noted, and thereafter cooling said molded sheet.

8. A method for forming sheets without a die from a plastic composed essentially of polytrifluorochloroethylene having an N. S. T value of about 300 C. which comprises placing the plastic between pressing platens containing a gauge block, compressing said plastic between said pressing platens at a pressure between 500 and about 1500 pounds per square inch to close said platens. against said gauge block at a temperature between about 500 F. and about 520 F. to mold said plastic into a sheet, after closing said platens against said gauge block to form the molded sheet maintaining said temperature and pressure at least until a color change indicative of plastification is noted, and thereaiter cooling 7 said molded sheet.

LOUIS C. RUBIN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS ern Plastics, October 1948, pages 168, 170, 172. 

1. A METHOD FOR FORMING SHEETS WITHOUT A DIE FROM A THERMOPLASTIC POLYMER OF TRIFLUOROCHLOROETHYLENE WHICH COMPRISES PLACING THE PLASTIC BETWEEN PRESSING PLATENS CONTAINING A GAUGE BLOCK, COMPRESSING SAID PLASTIC BETWEEN SAID PRESSING PLATENS UNDER SUFFICIENT ELEVATED PRESSURE TO CLOSE SAID PLATENS AGAINST SAID GAUGE BLOCK AT AN ELEVATED TEMPERATURE SUFFICIENT TO MOLD SAID PLASTIC INTO A SHEET, AFTER CLOSING SAID PLATENS AGAINST SAID GAUGE BLOCK TO FORM THE MOLDED SHEET MAINTAINING THE ELEVATED TEMPERATURE AND PRESSURE AT LEAST UNTIL A COLOR CHANGE INDICATIVE OF PLASTIFICATION IS NOTED, AND THEREAFTER COOLING SAID MOLDED SHEET. 